Apply concrete suggestions from code review

Fixes suggested by @ulysses4ever, thanks@

Co-authored-by: Artem Pelenitsyn <a.pelenitsyn@gmail.com>

doc/setup-commands.rst

@@ -882,7 +882,7 @@ Miscellaneous options
     Restrict solutions involving a package to given version
     bounds, flag settings, and other properties.
 
-    To following considers only install plans where ``bar``,
+    The following considers only install plans where ``bar``,
     if used, is restricted to version 2.1:
 
     ::
@@ -902,7 +902,7 @@ Miscellaneous options
         Often useful for GHC-supplied packages in combination with :cfg-field:`allow-newer`,
         e.g., ``--allow-newer='*:base' --constraint='base installed'``.
 
-      - ``source`` to a fix a package to the local source copy.
+      - ``source`` to fix a package to the local source copy.
 
     ::
 
@@ -928,7 +928,7 @@ Miscellaneous options
         # Require that bar have test suites and benchmarks enabled.
         $ cabal install --constraint="bar test" --constraint="bar bench"
 
-    Observe: To specify multiple constraints, you may pass the
+    To specify multiple constraints, you may pass the
     ``constraint`` option multiple times.
 
     By default, constraints only apply to build dependencies