doc/source/core_plugins.rst, doc/source/install_main.rst: Fixing link anchor name

In core_plugins.rst, we are already using _plugins, _plugins_build_elements,
so lets call this one _plugins_sources to be consistent.

doc/source/core_plugins.rst

@@ -42,7 +42,7 @@ Build elements
    elements/pip
 
 
-.. _authoring_source_plugins:
+.. _plugins_sources:
 
 Sources
 -------

doc/source/install_main.rst

@@ -14,7 +14,7 @@ BuildStream requires the following base system requirements:
 * psutil python library (so you don't have to install GCC and python-devel to build it yourself)
 
 BuildStream also depends on the host tools for the :mod:`Source <buildstream.source>` plugins.
-Refer to the respective :ref:`authoring_source_plugins` documentation for host tool
+Refer to the respective :ref:`source plugin <plugins_sources>` documentation for host tool
 requirements of specific plugins.
 
 If you intend to push built artifacts to a remote artifact server,