Corrected typo for print bed command "offset", expected parameter is called "offset" not "offsets"

Compare #444

docs/api/printer.rst

@@ -506,7 +506,7 @@ Issue a bed command
    offset
      Sets the given temperature offset on the printer's tools. Additional parameters:
 
-     * ``offsets``: Offset to set.
+     * ``offset``: Offset to set.
 
    All of these commands may only be sent if the printer is currently operational. Otherwise a :http:statuscode:`409`
    is returned.

src/octoprint/server/api/printer.py

@@ -168,7 +168,7 @@ def printerBedCommand():
 
 	##~~ temperature offset
 	elif command == "offset":
-		offset = data["offsets"]
+		offset = data["offset"]
 
 		# make sure the offset is valid
 		if not isinstance(offset, (int, long, float)):