Update bash.md (#1917)

* Update bash.md

declare variable as integer, then increment

* Update bash.md

Co-authored-by: Rico Sta. Cruz <rstacruz@users.noreply.github.com>

bash.md

@@ -637,6 +637,11 @@ $((a + 200))      # Add 200 to $a
 $(($RANDOM%200))  # Random number 0..199
 ```
 
+```bash
+declare -i count  # Declare as type integer 
+count+=1          # Increment
+```
+
 ### Subshells
 
 ```bash