docs: correct the LLVM feature flag naming in README (#707)

* docs: correct the LLVM feature flag naming in README

The README said the feature flag is llvmM-0 across LLVM 12-22, but
Cargo.toml uses llvmM-0 only for LLVM 12-17; LLVM 18-22 use llvmM-1.

The toml snippet three lines above already shows features = ["llvm22-1"],
so the example and the sentence describing it disagreed.

* docs: describe llvm feature flag suffix as LLVM's own major.minor version

Per maintainer feedback: the suffix isn't an arbitrary 0/1 split, it
matches LLVM's own minor version (e.g. llvm18-1 for LLVM 18.1.x).

---------

Co-authored-by: Socialpranker <0630039863abc@gmail.com>

Author: Socialpranker

README.md

@@ -26,7 +26,7 @@ inkwell = { version = "0.10.0", features = ["llvm22-1"] }
 ```
 
 Supported versions:
-LLVM 12-22 mapping to a cargo feature flag `llvmM-0` where `M` corresponds to the LLVM major version.
+LLVM 12-22 mapping to a cargo feature flag `llvmM-N` where `M` and `N` correspond to the LLVM major and minor version, e.g. `llvm18-1` for LLVM 18.1.x.
 
 Please be aware that we may make breaking changes on master from time to time since we are
 pre-v1.0.0, in compliance with semver. Please prefer a crates.io release whenever possible!