Update build concurrency numbers for Business (#7794)

This matches them with reality :)

Author: ericholscher

docs/builds.rst

@@ -23,7 +23,7 @@ Our current build limits are:
 
       * 30 minutes build time
       * 7GB of memory
-      * 4 concurrent builds
+      * Concurrent builds vary based on your pricing plan
 
       If you are having trouble with your documentation builds,
       you can reach our support at [email protected].
@@ -80,7 +80,7 @@ The *Sphinx* and *Mkdocs* builders set the following RTD-specific environment va
 .. csv-table:: Environment Variables
    :header: Environment variable, Description, Example value
    :widths: 15, 10, 30
-    
+
    ``READTHEDOCS``, Whether the build is running inside RTD, ``True``
    ``READTHEDOCS_VERSION``, The RTD name of the version which is being built, ``latest``
    ``READTHEDOCS_PROJECT``, The RTD slug of the project which is being built, ``my-example-project``