Update titles

docs/2_coordinates.md

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-title: Coordinates
+title: Coordinate systems
 ---
 # 2. Coordinate systems
 

docs/5_operators_QM.md

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-title: Operators quantum mechanics
+title: Operators in quantum mechanics
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 # 5. Operators in quantum mechanics

docs/7_differential_equations_1.md

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-title: Differential Equations
+title: Differential Equations: Part 1
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 #7. Differential equations: Part 1

docs/8_differential_equations_2.md

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-title: 8. Differential equations: Part 2
+title: Differential equations: Part 2
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 #8. Differential equations: Part 2