fix typo

README.md

@@ -39,7 +39,7 @@ Status of the chapters:
   4. :x: [Representing numbers as sums of two squares](./src/chapters/04_Representing_numbers_as_sums_of_two_squares.lean)
   5. :thought_balloon: [The law of quadratic reciprocity](./src/chapters/05_The_law_of_quadratic_reciprocity.lean)
   6. :thought_balloon: [Every finite division ring is a field](./src/chapters/06_Every_finite_division_ring_is_a_field.lean)
-  7. :thought_ballon: [The spectral theorem and Hadamard's determinant problem](./src/chapters/07_The_spectral_theorem_and_Hadamard's_determinant_problem.lean)
+  7. :thought_balloon: [The spectral theorem and Hadamard's determinant problem](./src/chapters/07_The_spectral_theorem_and_Hadamard's_determinant_problem.lean)
   8. :thought_balloon: [Some irrational numbers](./src/chapters/08_Some_irrational_numbers.lean)
   9. :x: [Four times ](./src/chapters/09_Four_times_pi²_over_6.lean)$\pi^2/6$