README.md: Fix MD040 warnings

The following warnings are generated by using markdownlint analysis:
```
MD040/fenced-code-language Fenced code blocks should have a language specified [Context: "```"]
```
Refer to markdown-lint [1] to fix MD040
[1]: https://github.com/updownpress/markdown-lint/blob/master/rules/040-fenced-code-language.md

Signed-off-by: George Liu <liuxiwei@ieisystem.com>
Change-Id: Ia2d19b88db5d82492f0b9a421f25bb933f041446
diff --git a/README.md b/README.md
index 41875b9..3a3b80e 100644
--- a/README.md
+++ b/README.md
@@ -22,7 +22,7 @@
 
 A typical dbus-sensors object support the following dbus interfaces:
 
-```
+```text
 Path        /xyz/openbmc_project/sensors/<type>/<sensor_name>
 
 Interfaces  xyz.openbmc_project.Sensor.Value
@@ -87,7 +87,7 @@
 your system-specific dts you would enable and configure what you want with
 something like this:
 
-```
+```text
 iio-hwmon {
     compatible = "iio-hwmon";
     io-channels = <&adc0 0>;
@@ -111,7 +111,7 @@
 You will then create an entity-manager configuration file that is of type "ADC"
 A very simple example would like look this:
 
-```
+```text
             "Index": 0,
             "Name": "P12V",
             "PowerState": "Always",