[llvm] [llvm] Proofread ProgrammersManual.rst (PR #150787)

[Tim Gymnich via llvm-commits]

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@@ -1908,38 +1908,38 @@ llvm/ADT/StringRef.h
 The StringRef class is a simple value class that contains a pointer to a
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tgymnich wrote:

```suggestion
The ``StringRef`` class is a simple value class that contains a pointer to a
```
missing ``?

https://github.com/llvm/llvm-project/pull/150787