osl-languagespec.pdf no longer maintained/outdated references

[Sparsh-N]

Theres a few small references to tex based documentation which was recently removed to avoid duplication in the docs. In the main README.md, under the Documentation section there's mention of osl-languagespec.pdf which goes as follows:

The OSL language specification can be found at
[src/doc/osl-languagespec.pdf](src/doc/osl-languagespec.pdf) (in a source
distribution) or in the share/doc/OSL/osl-languagespec.pdf file of an
installed binary distribution.

It is compiled when building the docs locally in src/doc/Makefile, although latex for the languagespec is dropped in #1970 to reduce confusion regarding which file to update.

languagespec.pdf: *.tex Figures/*.pdf Figures/*png
	pdflatex ${PDFLATEX_FLAGS} languagespec.tex
languagespec_index:
	pdflatex ${PDFLATEX_FLAGS} languagespec.tex
	makeindex languagespec
	pdflatex ${PDFLATEX_FLAGS} languagespec.tex languagespec.aux macros.aux
	pdflatex ${PDFLATEX_FLAGS} languagespec.tex languagespec.aux macros.aux languagespec.toc

A solution could be to modify the docs Makefile to go from .md -> .pdf, in place of the existing .tex -> .pdf. Alternatively, all mentions could be removed entirely if maintaining a pdf of the languagespec isn't needed.