Fix markdown format error

Run markdownlint, the following warning message appears:
```
README.md:15 MD040/fenced-code-language Fenced code blocks should
             have a language specified [Context: "  ```"]
README.md:22 MD040/fenced-code-language Fenced code blocks should
             have a language specified [Context: "  ```"]
README.md:29 MD040/fenced-code-language Fenced code blocks should
             have a language specified [Context: "  ```"]
README.md:47 MD040/fenced-code-language Fenced code blocks should
             have a language specified [Context: "  ```"]
README.md:58 MD040/fenced-code-language Fenced code blocks should
             have a language specified [Context: "  ```"]
README.md:88 MD040/fenced-code-language Fenced code blocks should
             have a language specified [Context: "  ```"]
README.md:103 MD040/fenced-code-language Fenced code blocks should
             have a language specified [Context: "  ```"]
```

Signed-off-by: George Liu <liuxiwei@inspur.com>
Change-Id: Id48ba2ebe3c82982f8fcc5d2029324f129249fd7
diff --git a/README.md b/README.md
index 4722680..dde465b 100644
--- a/README.md
+++ b/README.md
@@ -12,21 +12,21 @@
 
 - Compile phosphor-time-manager with default options:
 
-  ```
+  ```bash
      meson setup builddir
      ninja -C builddir
   ```
 
 - Compile phosphor-time-manager with some configurable options:
 
-  ```
+  ```bash
      meson setup builddir -Dbuildtype=minsize  -Dtests=disabled
      ninja -C builddir
   ```
 
 - Generate test coverage report:
 
-  ```
+  ```bash
      meson setup builddir -Db_coverage=true -Dtests=enabled
      ninja coverage -C builddir test
   ```
@@ -44,7 +44,7 @@
 
 - To get BMC's time:
 
-  ```
+  ```bash
   ### With busctl on BMC
   busctl get-property xyz.openbmc_project.Time.Manager \
       /xyz/openbmc_project/time/bmc xyz.openbmc_project.Time.EpochTime Elapsed
@@ -55,7 +55,7 @@
 
 - To set BMC's time:
 
-  ```
+  ```bash
   ### With busctl on BMC
   busctl set-property xyz.openbmc_project.Time.Manager \
       /xyz/openbmc_project/time/bmc xyz.openbmc_project.Time.EpochTime \
@@ -85,7 +85,7 @@
 
 - To set an NTP [server](https://tf.nist.gov/tf-cgi/servers.cgi):
 
-  ```
+  ```bash
   ### With busctl on BMC
   busctl set-property  xyz.openbmc_project.Network \
      /xyz/openbmc_project/network/eth0 \
@@ -100,7 +100,7 @@
 
 - To go into NTP mode
 
-  ```
+  ```bash
   ### With busctl on BMC
   busctl set-property xyz.openbmc_project.Settings \
       /xyz/openbmc_project/time/sync_method xyz.openbmc_project.Time.Synchronization \