cellsim help: apostrophes instead of backticks in fep-hyd description

Backticks in a bash heredoc get command-substituted, so
"cellsim fep-freesolv" (in backticks) was being executed by
bash and its output — nothing — replaced the literal text.
Render was an empty line where the cross-reference should be.

Swapped to single quotes ('cellsim fep-freesolv') — literal,
no substitution. The other pre-existing backticks are inside
"#" shell comments, which never reach the heredoc, so this is
the only call site that needed fixing.

Author: Rockman6

scripts/cellsim

@@ -88,7 +88,7 @@ usage: cellsim <subcommand> [args...]
 
   fep-hyd     alchemical hydration ΔG for a single SMILES —
                 scaffold-only primitive; real hydration gate is
-                `cellsim fep-freesolv`
+                'cellsim fep-freesolv'
                 -> src/fep/__init__.py
 
   fep-freesolv FreeSolv 12-compound hydration accuracy gate