Improve docs

- Fix typos

- Follow markdown linter to improve style:
  * Remove redundant empty lines
  * Insert empty lines for readability
  * remove trailing spaces

Author: ShapelessCat

CONTRIBUTING.md

@@ -10,7 +10,7 @@ abide by the [Rust code of conduct], which you can find at that link or in the
 
 ## License
 
-RBE is dual licenced under the MIT and Apache 2.0 licenses, and so are all
+RBE is dual licensed under the MIT and Apache 2.0 licenses, and so are all
 contributions. Please see the [`LICENSE-MIT`] and [`LICENSE-APACHE`] files in
 this directory for more details.
 
@@ -55,7 +55,7 @@ $ mdbook build
 
 **The following warnings can be ignored safely.**
 
-```
+```text
 [WARN] (mdbook::preprocess::cmd): The command wasn't found, is the "gettext" preprocessor installed?
 [WARN] (mdbook::preprocess::cmd):   Command: mdbook-gettext
 ```

README.md

@@ -26,7 +26,7 @@ read all content offline, however!
 
 **The following warnings can be ignored safely.**
 
-```
+```text
 [WARN] (mdbook::preprocess::cmd): The command wasn't found, is the "gettext" preprocessor installed?
 [WARN] (mdbook::preprocess::cmd):   Command: mdbook-gettext
 ```