_VARIABLES.md: add missing 'be' into the sentence

Closes #12809

docs/cmdline-opts/_VARIABLES.md

@@ -5,7 +5,7 @@ curl supports command line variables (added in 8.3.0). Set variables with
 --variable name=content or --variable name@file (where "file" can be stdin if
 set to a single dash (-)).
 
-Variable contents can expanded in option parameters using "{{name}}" (without
+Variable contents can be expanded in option parameters using "{{name}}" (without
 the quotes) if the option name is prefixed with "--expand-". This gets the
 contents of the variable "name" inserted, or a blank if the name does not
 exist as a variable. Insert "{{" verbatim in the string by prefixing it with a