doc: Fixes typo in variable name in README.

Change-Id: Ia76167fc6c4e9ac6be6356c7ce08878c7adc7fbf
Reviewed-on: http://gerrit2.aldebaran.lan/1370
Reviewed-by: jmonnon <jmonnon@aldebaran.com>
Reviewed-by: philippe.martin <philippe.martin@softbankrobotics.com>
Tested-by: vincent.palancher <vincent.palancher@softbankrobotics.com>

Author: Vincent Palancher

README.rst

@@ -122,7 +122,7 @@ The project's dependencies on LibQi depends on the building mode:
   - `LIBQI_GIT_TAG`
 
 It is possible to skip the download step and use an existing source directory by
-setting its path as the `FETCH_CONTENT_SOURCE_DIR_LIBQI` CMake variable. The
+setting its path as the `FETCHCONTENT_SOURCE_DIR_LIBQI` CMake variable. The
 build system will still check that the version of the sources matches the
 `LIBQI_VERSION` value if it is set.