Fix documentation of environment variable defaults (grafana#6536)

For some reason, the `-` character was missing here in the documentation.
After struggling for some time, I finally figured out that -- quite obviously -- envsubst default behavior applies and thus the documentation was wrong. :-)

Author: headcr4sh

docs/sources/clients/promtail/configuration.md

@@ -55,7 +55,7 @@ References to undefined variables are replaced by empty strings unless you speci
 To specify a default value, use:
 
 ```
-${VAR:default_value}
+${VAR:-default_value}
 ```
 
 Where default_value is the value to use if the environment variable is undefined.