Fix cabal install commands for local HLS build in docs

[9999years]

The existing command errors:

$ cabal install pkg:haskell-language-server
cabal: Internal error in target matching. It should always be possible to find
a syntax that's sufficiently qualified to give an unambiguous match. However
when matching 'pkg:haskell-language-server' we found haskell-language-server
(named-package) which does not have an unambiguous syntax. The possible syntax
and the targets they match are as follows:
'haskell-language-server' which matches haskell-language-server
(named-package)
'pkg:haskell-language-server' which matches haskell-language-server
(named-package), pkg:haskell-language-server (unknown-component),
:pkg:pkg:lib:pkg:file:haskell-language-server (unknown-file)
':pkg:haskell-language-server' which matches haskell-language-server
(named-package)

[fendor]

Thank you for catching this!