Fix typo in section 'Standards for Example Code' in 'contributing.md' (#2039)

Author: yvonnefroehlich

doc/contributing.md

@@ -422,7 +422,7 @@ When editing documentation, use the following standards to demonstrate the examp
 3. Optional arguments are wrapped with [ ] (square brackets).
 4. Arguments that are mutually exclusive are separated with a | (bar) to denote "or".
 5. Default arguments for parameters and configuration settings are wrapped
-   with [ ] (square brackers) with the prefix "Default is". Example: [Default is
+   with [ ] (square brackets) with the prefix "Default is". Example: [Default is
    **p**].
 
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