gh-40348: Fix a small typo
    
change `bool` to `positive`. And reformat some `%` to f-string.

I don't have gap3 to test, but this looks simple enough that surely it
can't go wrong (?)


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URL: #40348
Reported by: user202729
Reviewer(s): Frédéric Chapoton

Author: Release Manager

src/sage/combinat/root_system/reflection_group_element.pyx

@@ -812,10 +812,10 @@ cdef class RealReflectionGroupElement(ComplexReflectionGroupElement):
             False
         """
         if not isinstance(positive, bool):
-            raise TypeError("%s is not a boolean" % (bool))
+            raise TypeError(f"{positive} is not a boolean")
 
         if i not in self._parent.index_set():
-            raise ValueError("the given index %s is not in the index set" % i)
+            raise ValueError(f"the given index {i} is not in the index set")
 
         negative = not positive