format the "generation" subtitles (#23517)

Format the "generation" subtitles so they all have the same spacing.

guide/english/computer-hardware/gpu/index.md

@@ -34,9 +34,9 @@ GPUs were originally modeled on the concept of graphics pipeline. Graphics pipel
 
 ### 0th Generation
 "Reality Engine" board by Silicon Graphics Inc.(SGI) marked the onset of GPU hardware and the graphics pipeline. But the technology was still dependent upon CPU for the first half. Also, the speed was limited to one pixel execution per clock cycle. The engine use OpenGL, a widely used 2D/3D application programming.
-### 1stGeneration
+### 1st Generation
 The "3dfx Voodoo" (1996) evolved as one of the first true 3D-accelerator for games. It handled texture mapping, rasterization, and z-buffering but the CPU still had to do vertex transformations.
-### 2ndGeneration
+### 2nd Generation
 This is the point when the first-ever true GPU, NVIDIA's "GeForce 256" was released in the common market. The GPUs of this generation's used Accelerated Graphics Port(AGP), offered new functions like multi-texturing, hardware geometry transform, light maps, and lighting. The traditional pipelines were known as a "fixed function" pipeline, because once the developer sent graphics data into the GPU's pipeline, the data could not be changed.
 ### 3rd Generation
 With this generation of CPUs, programmable pipelining came into existence. Now the previously non-programmable parts could be programmed by programmers. In 2001, NVIDIA released the GeForce3.