tools: Fix typo in comment

Author: Patater

tools/toolchains/mbed_toolchain.py

@@ -652,7 +652,7 @@ def is_not_supported_error(self, output):
 
     @abstractmethod
     def parse_output(self, output):
-        """Take in compiler output and extract sinlge line warnings and errors from it.
+        """Take in compiler output and extract single line warnings and errors from it.
 
         Positional arguments:
         output -- a string of all the messages emitted by a run of the compiler