Address example use of sh variable (#1209)

Author: kurtmckee

src/globus_cli/commands/mkdir.py

@@ -17,8 +17,8 @@
 
 [source,bash]
 ----
-$ ep_id=aa752cea-8222-5bc8-acd9-555b090c0ccb
-$ mkdir ep_id:~/testfolder
+$ EP_ID=aa752cea-8222-5bc8-acd9-555b090c0ccb
+$ mkdir $EP_ID:~/testfolder
 ----
 """,
 )