docs: fix invalid function-call example and inaccurate escape-standard reference

[prql-bot]

Two documentation corrections found during the nightly survey of web/book/src/reference/syntax/.

1. function-calls.md — invalid function-call example. The nested-call example used a comma to separate the inner arguments:

outer_func arg_1 (inner_func arg_a, arg_b) arg_2

PRQL function arguments are whitespace-separated; a comma there is a parse error ((math.round 1, 2) fails, (math.round 1 2) compiles). The example is in a prql no-eval block, so the doc-test harness never caught it. Removed the comma.

2. strings.md — inaccurate escape-standard reference. The text claimed strings "can contain any escape character sequences defined by the [JSON standard]", but the examples immediately below use \x48 (C-style hex) and \u{01F422} (brace/6-digit) escapes — neither of which JSON defines (JSON only allows 4-digit \uXXXX and has no \x). The document's own "Escape sequences" section already, correctly, describes them as "rules similar to those used by Standard C". Changed the earlier sentence to match and to link the detailed section rather than the (incorrect) JSON reference.

No regression test: both are documentation-only changes. The function-calls.md snippet is no-eval by design, and the strings.md change is prose.