Make a minor formatting change to the help page for jj parallelize

Per discussion on another PR, we're going to avoid ALL CAPS for argument names
in the documentation even though Clap uses that style for the `--help` output.

- #3453 (comment)

cli/src/commands/parallelize.rs

@@ -46,7 +46,7 @@ use crate::ui::Ui;
 /// the target revset (not to be confused with the repo root). The children of
 /// the head(s) of the target revset are rebased onto the target revisions.
 ///
-/// The target revset is the union of the REVISIONS arguments and must satisfy
+/// The target revset is the union of the `revisions` arguments and must satisfy
 /// several conditions, otherwise the command will fail.
 ///
 /// 1. The heads of the target revset must have either the same children as the

cli/tests/cli-reference@.md.snap

@@ -1366,7 +1366,7 @@ Each of the target revisions is rebased onto the parents of the root(s) of
 the target revset (not to be confused with the repo root). The children of
 the head(s) of the target revset are rebased onto the target revisions.
 
-The target revset is the union of the REVISIONS arguments and must satisfy
+The target revset is the union of the `revisions` arguments and must satisfy
 several conditions, otherwise the command will fail.
 
 1. The heads of the target revset must have either the same children as the