CONTRIBUTING.md: grammar and strengthen wording

https://app.grammarly.com has a free grammar checker.
It flagged several things in the CONTRIBUTING file, fixed some
them.
Also strengthened the wording around using system() from almost
never to do not use.

Learn something new every day:
https://www.dailywritingtips.com/that-vs-which/

Change-Id: Ic12468761866ee2165b324f0c87245369f79e0d3
Signed-off-by: Gunnar Mills <gmills@us.ibm.com>
diff --git a/CONTRIBUTING.md b/CONTRIBUTING.md
index 689cc27..f8f7018 100644
--- a/CONTRIBUTING.md
+++ b/CONTRIBUTING.md
@@ -37,8 +37,8 @@
 
 Performing code reviews is another good way to get started.  Go to
 https://gerrit.openbmc-project.xyz and click on the "all" and "open"
-menu items, or if you are interested in a particular repository, for
-example "bmcweb", type "status:open project:openbmc/bmcweb" into the
+menu items, or if you are interested in a particular repository - for
+example, "bmcweb" - type "status:open project:openbmc/bmcweb" into the
 search bar and press the "search" button.  Then select a review.
 Remember to be positive and add value with every review comment.
 
@@ -63,7 +63,7 @@
 requirements. This enforces PEP 8 standards on all Python code.
 
 OpenBMC standards for Python match with PEP 8 so in general, a blank setup.cfg
-file is all that's needed. If so desired, an enforcement of 80
+file is all that's needed. If so desired, enforcement of 80
 (vs. the default 79) chars is fine as well:
 ```
 [pycodestyle]
@@ -194,7 +194,7 @@
 After signing a CLA, send it to openbmc@lists.ozlabs.org.
 
 _One time setup_: Login to the WebUI with your GitHub credentials and verify on
-your account Settings that your SSH keys were imported:
+your Account Settings that your SSH keys were imported:
 
   https://gerrit.openbmc-project.xyz/#/settings/
 
@@ -237,7 +237,7 @@
 or menu (All > Open, or My > Changes).
 
 Invite reviewers to review your changes.  Each OpenBMC repository has
-a `MAINTAINERS` file which lists required reviewers who are subject
+a `MAINTAINERS` file that lists required reviewers who are subject
 matter experts.  Those reviewers may add additional reviewers.  To add
 reviewers from the Gerrit web page, click the "add reviewers" icon by
 the list of reviewers.
@@ -289,8 +289,8 @@
 There are numerous resources available elsewhere, but a few items that are
 relevant to OpenBMC work:
 
- * You almost never need to use `system(<some shell pipeline>)`. Reading and
-   writing from files should be done with the appropriate system calls.
+ * Do not use `system(<some shell pipeline>)`. Reading and writing from
+   files should be done with the appropriate system calls.
 
    Generally, it's much better to use `fork(); execve()` if you do need to
    spawn a process, especially if you need to provide variable arguments.